1. Field of the Invention
This invention relates to signal processors, and more particularly, to a method and apparatus for noise burst detection in signal processors.
2. Description of Related Art
Thanks to the advances in semiconductor technologies, many electronic devices, such as digital cordless telephones, can be now made compact but nonetheless provide an increased level of functionality with high integration and low cost. The digital cordless telephone includes a handset which is provided with a microphone for receiving and converting a human voice into an analog speech signal. The analog speech signal is subsequently converted into digital form which is compressed and then modulated into a radio frequency (RF) signal for transmission by means of an antenna through the ether to a nearby base station. The base station serves as a relay to retransmit the RF signal to the destination telephone set. Upon receiving the RF signal, the circuitry in the destination telephone set first demodulates the RF signal into digital form, then decompresses the digital data, converts the digital data into analog form, and finally broadcasts the analog signal (which represents the original voice of the caller) on a speaker on the destination telephone set.
The UK (United Kingdom) Cordless-Telephone Second Generation CT-2, for example, sets forth a standard for cordless telephone communications. In compliance with the CT-2 standard, the handset on the caller's site will process the analog speech signal generated by the microphone by means of the so-called Adaptive Differential Pulse Code Modulation (ADPCM) encoder for pulse code modulation (PCM) of the analog speech signal into digital form, and then transmit the modulated signal via the so-called Common Air Interface (CAI) to the base station.
The handset and the base station are continuously linked until the communication therebetween is over. In some circumstances, for instance when the handset is moved beyond the reception range of the base range, the link is broken. When this happens, high volume white noise will be output to the speaker. This noisy broadcast of the white noise is usually annoying to the user. The speaker will be muted only until the handset detects that the link is disconnected. To detect whether the link is connected or disconnected, the CT-2 standard sets forth a so-called "shake-hand" signal which is embedded in the transmitted signal for the handset to determine whether the link is still connected. One drawback to this solution, however, is that the interval of the shake-hand signal will last for several seconds, which is quite a long duration for the user to tolerate the noisy broadcast from the speaker on the handset. A conventional solution can reduce the interval of the shake-hand signal down to some hundreds of milliseconds (ms). However, users still consider this period too long to tolerate the noisy broadcast from the speaker on the handset when the link is broken.
Various noise burst detection techniques have been proposed for use on digital signal systems, such as the above-mentioned ADPCM-based digital communication system, for quick detection of the appearance of noise burst to thereby prevent them from being broadcast on the speaker on the handset. Conventional noise burst detection techniques include, for example, the U.S. Pat. No. 5,317,522 to Bonnet et al. and the U.S. Pat. No. 5,319,573 to Corleto et al., which are particularly used on ADPCM encoders to detect noise burst and, in the event of the occurrence of a noise burst, shut off the speaker so as to mute the noise.
However, it is a drawback of these two patents that, since multiplexers and accumulators are required for signal processing to determine whether the received signal is noise or a normal signal, the hardware is very complex. The manufacturing cost of the two patents is therefore high.